A large number of photonic structures and devices either benefit from, or require, active modulation of their optical response. A metal film providing a near-uniform lateral voltage (current) distribution represents an ideal electrical contact for such electro-optical devices. Unfortunately, the same free electrons that are responsible for the high DC conductivity of metals also dominate their optical properties, causing metals to be highly reflective at optical frequencies. In addition to their traditional role as electrical contacts for electro-optic devices, metals are of increasing interest for their optical properties. The wide and varied field of plasmonics, for instance, is in large part geared towards leveraging the ability of metal/dielectric structures to confine light to subwavelength volumes, thus enhancing light-matter interaction, and enabling next-generation nanophotonic devices. Yet, here too, the use of metals comes with a cost, as parasitic absorption of light via (ohmic) losses in plasmonic materials, in addition to strong reflection, limit the functionality of many plasmonic structures. Thus, the integration of metal into any optical or optoelectronic structure or device, while often providing very real benefits (subwavelength confinement, uniform electrical contact, etc.) is almost always accompanied by absorption and reflection losses compromising the ultimate performance of the optical structure or device.
Transmission of light through a smooth interface between two materials can be related to the change of material permittivity via the Fresnel equations. However, this simple relationship is violated in structured composites. All-dielectric “moth-eye” interfaces are known to reduce the reflectivity between two dielectric media by creating a surface layer with gradually changing refractive index. Nanostructuring, or even simple roughening, of semiconducting solar cell material can efficiently scatter incident radiation, increasing path length for light in the detector structure and acting as an anti-reflection coating. However, for the development of active devices, structuring the dielectric interface does little to enable efficient electrical contact, which requires the integration of (often, highly reflective) conducting material with the devices' active dielectric components.
The optical response of reflecting structured metallic films can be modified by coupling the incident radiation to a special type of highly-confined electromagnetic waves supported by thin metal films, surface plasmon polaritons (SPPs), followed by the out-coupling of SPPs into the dielectric on the other side of the film. Remarkably, the percentage of light transmitted through such structured metal films can exceed, at select frequencies, the percentage of open area in the films, a phenomenon known as extraordinary optical transmission (EOT), a source of substantial interest in the optics community since the initial demonstration of EOT nearly two decades ago. More recent research, aimed at elucidation of the origin of EOT, has provided a number of complex coupled (and sometimes competing) mechanisms, related to the excitation, transmission, and out-coupling of i) SPPs at the two metal-dielectric interfaces and ii) waveguide modes supported by the openings in the perforated metal films.